The present invention relates to a (meth)acrylate metal salt-containing rubber composition comprising a plasticiser having one or more epoxide groups per molecule.
Acrylate or methacrylate metal salts are added to rubber compositions prior to curing to impart to the cured product qualities such as increased stiffness, hardness and abrasion resistance. Acrylate or methacrylate metal salts are used in compositions with any elastomer that can be polymerized with a peroxide initiator. The acrylate or methacrylate salt presumably participates in cross-linking of the elastomer during curing, and also is believed to form a poly-metal-acrylate or poly-metal-methacrylate matrix throughout the rubber. The most commonly used salts are zinc diacrylate and zinc dimethacrylate. Cured rubber compositions comprising acrylate or methacrylate metal salts are suitable for use in applications where a particularly stiff and abrasion resistant rubber is required. Typical end uses include conveyor belts, shoe bottoms, hard roll covers and golf balls. For use in conveyor belts, a typical rubber composition comprises hydrogenated acrylonitrile-butadiene rubber (NBR) as the main elastomer component, zinc diacrylate, and a plasticizer. A typical composition for golf balls comprises polybutadiene as the elastomer and zinc diacrylate.
Rubber compositions formulated with acrylate and methacrylate metal salts suffer the considerable drawback of emitting an offensive odour during processing. The odour can be offensive enough to make processing extremely unpleasant, at best. Some processing companies have reported that processing of this kind of composition is impossible without the introduction of special ventilating means. The intensity of the odour increases when the composition is heated, but is detectable at ambient temperatures. The odour has also been observed to be more severe when a rubber composition containing an acrylate or methacrylate metal salt is stored for long periods (typical storage may be for as long as two years). It would be of significant value to find a means for reducing or eliminating the odour emitted from acrylate and methacrylate metal salt-containing compositions.
Plasticisers, or softeners, are compounds which are added to rubber compositions to improve processing characteristics, and often to produce a finished product with altered characteristics. Plasticisers are usually organic compounds that act as high boiling solvents for the elastomer component of the rubber composition. When added to an elastomer, they soften the resulting composition. In a rubber composition, softening is reflected by a decreased Mooney viscosity. This leads to greater ease of mixing, and improvement of other processing characteristics (for example, ease of shaping, moulding, blowing, etc.). When cured, rubber compositions containing plasticisers usually have a decreased modulus and a decreased hardness relative to the analogous cured composition without plasticiser. In rubber compositions destined for end uses that require a high modulus or a high degree of hardness, the addition of a conventional plasticisers to the rubber composition is therefore detrimental. As mentioned above, rubber compositions which contain no plasticiser are usually extremely difficult to process. A plasticiser which confers a balance of good processing characteristics and high modulus and high degree of hardness in the cured product is highly desirable.
In one aspect the invention provides a curable rubber composition comprising an acrylate or methacrylate metal salt, an elastomer that is curable with a peroxide initiator, and a plasticizer having one or more epoxide groups per molecule.
In another aspect, the invention provides a cured rubber composition prepared by adding a peroxide initiator to a curable rubber composition of the invention and heating the resulting mixture to a temperature sufficient to cure the composition.
In another aspect, the invention provides a process for preparing a curable rubber composition, the process comprising compounding an acrylate or methacrylate metal salt, an elastomer that is curable with a peroxide initiator, and a plasticizer having one or more epoxide groups per molecule.
In yet another aspect, the invention provides a process for preparing a cured rubber composition, the process comprising adding a peroxide initiator to a curable rubber composition of the invention, and heating the resulting mixture to a temperature sufficient to cure the composition. BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a gas-chromatography-mass-spectrometry (GCMS) trace for a sample containing the gaseous emissions from a composition of the prior art, when the composition is heated to 150xc2x0 C.;
FIG. 2 shows an expanded portion of the GCMS trace of FIG. 1;
FIG. 3 shows the maximum torque, as measured by a moving die rheometer (MDR), plotted against epoxidized soybean oil content in a zinc diacrylate/hydrogenated acrylonitrile-butadiene/peroxide composition of the invention;
FIG. 4 shows the 25, 50 and 100% modulus plotted against epoxidized soybean oil content in cured rubber compositions of the invention;
FIG. 5 shows in bar chart format the 100% modulus for cured rubber compositions containing various plasticisers;
FIG. 6 shows maximum torque as measured by the moving die rheometer (MDR) for cured rubber compositions containing various plasticisers;
FIG. 7 shows the composition Mooney viscosity for rubber compositions containing various plasticisers;
FIG. 8 shows the compound Mooney viscosity for rubber compositions containing various plasticisers; and
FIG. 9 shows the hardness for cured rubber compositions containing various plasticisers.